Most Arizonans share habitat with the mosquito that causes the Zika virus, whose possible link to babies born with small heads and brains in Brazil caused the World Health Organization to declare its spread a “global health emergency” on Monday.
So far, the Aedes aegypti mosquito has been simply an ankle-biting nuisance in Tucson, Phoenix and other urban areas of the state, despite its potential to spread the related viruses of Zika, Chikungunya and dengue fever.
Researchers at the University of Arizona have studied the pest for years, trying to puzzle out why the mosquito is endemic here but not the diseases it carries, despite their presence fairly nearby.
Outbreaks of dengue fever have occurred as close as Hermosillo, Sonora, 125 miles south of the Arizona border, but there have been no outbreaks in the border towns of Douglas or Nogales, and no reported local transmission of the diseases in Arizona.
Dengue outbreaks have occurred in the Texas/Chihuahua border towns of Brownsville and Matamoros in 2005 and in Key West, Florida, in 2010 and 2011.
Chikungunya has also been transmitted in Mexico, but not here.
Mexico has reported cases of locally transmitted Zika, but not in its border states.
Should we be worried about Zika?
Not right now. “Mosquito populations are small. It’s cold,” said Kacey Ernst, a UA epidemiologist who studies mosquito-borne diseases.
The recent outbreaks of Zika center on South America, where it’s summer and mosquito season. For now, the U.S. Centers for Disease Control and Prevention advises against travel to affected countries by pregnant women or women contemplating getting pregnant.
If and when Zika makes its way to Arizona, it will do so in summer, probably late summer after the monsoon rains create conditions for increases in the population of Aedes aegypti.
Even then, past experience with dengue and Chikungunya suggest that the disease will not get established here — though health officials say there is no certainty that we are somehow immune from outbreaks.
That, however, has been true so far with dengue and Chikungunya.
Ernst, an epidemiologist and biostatistician with the UA Mel and Enid Zuckerman College of Public Health since 2008, has concentrated her studies on dengue after earlier investigations of malaria in West Africa.
She said she at first thought lifestyle was the main reason for the lack of dengue’s spread northward. That was the case in Brownsville, where the 2005 dengue outbreak was mild compared to its cross-border neighbor.
As in Texas, most Arizonans live and work in screened, air-conditioned and mostly mosquito-free buildings.
In summer, when Aedes aegypti mosquitoes emerge after the monsoons, most of our daytime hours are spent indoors — in mosquito-free comfort. Most of our water is delivered in closed pipes. In Nogales, Sonora, said Ernst, 30 percent of the population collects rainwater, often in uncovered containers.
The chances of a mosquito biting an infected person and spreading the disease to an uninfected human by living long enough for the virus to make it to its salivary glands are lessened by our infrastructure.
But that does not explain the difference in disease spread between Hermosillo and Nogales. Ernst and UA entomologists Mike Riehle and Kathleen Walker continue to explore that angle, and in published studies suggest that climate plays a big role.
In an ongoing study funded by the National Institutes of Health, they have tracked the spread of the disease and collected mosquitoes from various locations.
Riehle and colleagues in the UA Department of Entomology developed a method for determining the age of the mosquitoes, discovering that the populations are older in outbreak areas.
Something in the climate of Tucson and Nogales is shortening the lifespan of Aedes aegypti or lengthening the incubation period needed to move the virus from its gut to its salivary glands.
“There is some evidence to believe the mosquito doesn’t live long enough here to transmit dengue. We’re at the edge of its environmental range. They don’t survive here the needed 10 to 14 days,” said Riehle.
Of course, climate varies from year to year.
In 2014, as a series of late summer storms hit Nogales, the team found much older populations of mosquitoes and some cases of local transmission.
Normally, Ernst suspects, Nogales, at an elevation of nearly 4,000 feet, is not warm enough to be conducive to dengue spread.
How will we know?
Health officials plan to use the heightened awareness of the potential for outbreaks to educate Arizonans on what we can do to prevent them.
The desert is not a welcome habitat for Aedes aegypti, but the mosquitoes find refuge in our yards — they hang out in shady, moist spots in irrigated shrubs and find convenient breeding spots in dog dishes, flowerpots and uncovered water-harvesting containers.
The mosquito doesn’t need much. A crease in a plastic trash bag left out in the rain provides ample space for eggs to hatch into larva.
“People have done a really good job of allowing these Aedes aegypti to exploit our man-made containers,” said Ernst.
Vigilance is required, and maybe a neighborly helping hand. The mosquitoes don’t travel far, but you need to eliminate breeding areas in your immediate neighborhood. “So help your elderly neighbor and dump their standing water for them,” said Ernst.
Jessica Rigler, chief of the bureau of epiemiology and disease control with the Arizona Department of Health Services, suggests wiping that bowl or pot in addition to emptying it. Aedes aeguypti are “floodwater mosquitoes.” They lay eggs at the water line of a container, so that their offspring hatch into larvae with the next rain or overwatering.
The best protection from any mosquito-borne disease is simply to not get bitten, she said. The state Health Department recommends wearing long clothing and using insect repellent outdoors during times mosquitoes are active.
Not getting bitten also provides a type of “herd immunity” akin to that afforded by vaccination programs, Rigler said.
Rigler said the Health Department must walk a fine line between assuring people that diseases are not present and urging prevention strategies, as it watches for outbreaks.
The state Health Department works with doctors and health officials to educate them about symptoms and collect reports of infection, but it does not have active surveillance programs of the Aedes aegypti population.
“There is this kind of issue with passive surveillance,” said Ernst. “You have to wait until somebody gets sick enough to go to the doctor. The doctor has to order the right tests and wait for the results.”
By the time health officials know the disease is present, the outbreak could be already underway.
Ernst is working with the CDC and the Skoll Global Threats Fund to develop a social-media tool that could serve as an early-warning system — a symptom-reporting app for mobile devices akin to one already in use for influenza. The three related diseases share common symptoms — high fever, headache, muscle and joint pain and sometimes nausea, vomiting and skin rash.
Ernst hopes to have the app ready for reporting of symptoms by June.
Outbreaks usually bring swift responses in this country. Riehle said when he and his fellow dengue researchers went to Key West during the 2011 outbreak to collect mosquitoes for aging, they arrived too late.
“They basically carpet-bombed the place with insecticide. Once we got there, the mosquitoes were few and far between. It blew a hole in our study.”
That’s the norm in the public-health world, said Heidi Brown, also of the UA School of Public Health.
That was certainly the case in the 1950s and 1960s when a nationwide response to the much deadlier yellow fever, also carried by Aedes aegypti, severely knocked back its population, she said.
Aedes aegypti basically disappeared from Arizona beginning in the early 1970s, only to re-emerge in Tucson in the early 1990s, when complaints from the midtown Sam Hughes Neighborhood led entomologist Henry Hagedorn to trap and identify the burgeoning Aedes aegypti population there.
But while the mosquitoes returned, yellow fever has not, and the United States has seen few local transmissions of dengue and Chikungunya.
Whether Zika will be different is still a question nobody can answer.
Ernst said the research literature about the virus is scarce.
She found 170 articles, compared to the “thousands upon thousands” of dengue studies.
Riehle said he’s not overly concerned. “The populations aren’t huge here. Dengue, Zika and Chikungunya aren’t here yet. If there were some infections, there would be a very robust reaction, a huge mosquito-control effort and it would be stamped out quickly. I wouldn’t worry about it.”
Contact reporter Tom Beal at firstname.lastname@example.org or 573-4158.